Five adult asthmatics completed five standardized exercise tests while inhaling aerosols with different osmolarities. The nebulized solutions were 2% NaCl (osmolarity = 616 mosm), 4% NaCl (osmolarity = 1232 mosm), 6.1% dextrose (osmolarity = 308 mosm = isoosmolar), 24.4% dextrose (osmolarity = 1232 mosm) and distilled water (osmolarity = 0). All the patients had EIA. During the study all conditions, except the osmolarity of the inhaled aerosols were kept constant. There was no statistical difference in the response to the exercise on the five days, the fall in PEF being 22.8% after exercise while inhaling 2% NaCl, 17.8% after inhaling 4% NaCl, 16.2 after inhaling 6.1% dextrose, 24.8% after inhaling 24.4% dextrose and 21.6% after exercise while inhaling nebulized distilled water, respectively. It is concluded that the osmolarity of the inhaled aerosol is of little or no importance in exercise-induced asthma.